Mechanistic Insights into the Structural, Electrochemical, and Charge-Storage Behavior of CNT@NiO Electrodes for High Capacitance and Durable Supercapacitors

Introduction

Although Li-batteries remain widely used, there is a shift toward alternative energy storage solutions, such as supercapacitors (SCs), which offer high power output, long lifespan, rapid charge–discharge performance, low cost, and eco-friendliness. Supercapacitors often use transition metal oxides, such as nickel oxide (NiO), due to their high theoretical specific capacitance of 2,584 F g^-1, abundance, eco-friendliness, and chemical stability. However, due to NiO’s poor conductivity and structural limitations, it is commonly combined with carbon nanotubes (CNTs), whose high conductivity and large surface area enhance the overall electrochemical performance.

Methods

Herein, spray pyrolysis, a straightforwardand cost-effective method, was employed to deposit pure NiO and a composite of NiO and CNT (CNT@NiO) at 350 °C for use as supercapacitor electrodes. The structures and morphologies of the materials were probed using X-ray diffraction (XRD), scanning electron microscopy (SEM), Fourier-transform infrared spectroscopy (FT-IR), and nitrogen adsorption/desorption. The electrochemical performance of the pristine and composite electrodes was tested by cyclic voltammetry, galvanostatic charge/discharge, and electrochemical impedance spectroscopy in 2 M KOH over a 0-0.7 V potential window.

Results

SEM analysis revealed that pristine NiO nanoparticles had an irregular shape, whereas the composite exhibited a cylindrical morphology. According to Brunauer–Emmett–Teller (BET) analysis, the specific surface area of the composite was measured at 46.51 m² g^-1, which is greater than the 29.55 m² g^-1 observed for pristine NiO, and the average pore diameter was 27.37 nm, surpassing NiO’s 6.10 nm. Electrochemical testing revealed that incorporating CNTs significantly improved the composite’s electrochemical performance. The CNT@NiO electrode supplied an exceptional specific capacitance of 472.4 F g^-1 at 2 A g^-1, far greater than pristine NiO’s 18.92 F g^-1. Additionally, the composite exhibits a 68.8% rate capability (13.6% for NiO) and 93.3% cyclic stability (81.5% for NiO) at 2 A g^-1 after 1,000 cycles, indicating its suitability for high-performance supercapacitors.

Conclusion

These favorable results point to two deductions: (i) CNT@NiO is a promising candidate for energy storage, and (ii) spray pyrolysis is an affordable and effective electrode material fabrication method. Two novelties regarding the use of spray pyrolysis as a fabrication technique are presented in this work: (i) an in-depth discussion on reasons for prolonged cycle life in connection with structural modifications, and (ii) investigation and comparison of the Faradaic and capacitive charge storage contributions of the synthesized materials. Future work will focus on detailed interfacial analysis (e.g., using TEM and XPS) and long-term cycling assessments (>5,000 cycles) in full-cell configurations to further validate the practical application potential. Overall, this study presents a viable path toward locally fabricated, efficient energy storage systems that can drive the adoption of clean energy in Africa and help bridge the global energy gap.